"I favor legalizing same-sex marriages, and would fight efforts to prohibit such marriages," the young candidate held, in 1996. On Wednesday, at the White House, he finally said about the same thing.

Perhaps, the vice-president did Obama a service: by rushing in, and by summoning the morning show host Robin Roberts down to Washington with the urgency of a country veterinarian at a calf birthing, the president looked braver than he ought after his years of kind-of, sort-of, not-really, wink-wink triangulation. But hedging as he had throughout his first term, while cabinet members and Democratic bigwigs jumped in front of him, had begun to obscure the major accomplishments Obama should get more credit for. Last year, he instructed Eric Holder, the attorney general, to end Justice Department support for the Defense of Marriage Act (Doma), which denies federal benefits to married gay couples. It was a far more consequential decision that the one he made this week – and yet, by not going on the record as himself in favor of gay marriage, he got only a fraction of the credit he deserved.

So, Obama has, belatedly, come out of the closet as a defender of equality. But the reaction – hosannas, banner headlines, tone-deaf invocations of "We shall overcome" – reveals more than anything how gay rights in the United States have become wholly contiguous with the marriage equality movement, and how gesture and affirmation matter more than action and justice. Nothing substantive has changed from last week to this one, but that didn't stop one New York Times columnist from mawkishly appending the hashtag #historymade to a tweet celebrating the move. In a way, it feels a bit like Obama's 2008 victory itself: electing a black president was "change", and the image of racial progress crowded out the true inequalities that blacks continue to endure.

Of course, the president's support has value – I myself was waiting for it. The president is the head of the body politic, and the more public and more widespread that support for gay marriage becomes in America, the more likely it is that the US supreme court and its swing justice, Anthony Kennedy, will strike the final blow for equality when Perry v Brown, or another case, finally reaches their docket. It was public sentiment, not any change in the practice of jurisprudence, that led the court to reverse itself on the constitutionality of sodomy laws when it decided Lawrence v Texas in 2003. (Even many gay people don't realize, though, that we won Lawrence not because the justices said gays are equal to straights; they ruled that all Americans have a right to privacy in sexual conduct. The court has never, not once, deemed gays a class worthy of "strict scrutiny", the highest level of equal protection.)

Public sentiment counts. When the justices finally hear arguments on marriage, it'll be much easier to make the right call if most Americans, from the president down, want to hear as much.

But marriage isn't everything. As with women and with racial minorities, gays in America continue to suffer from harassment, hate crimes, religious intolerance and a host of other inequities, which marriage cannot alleviate. Gays can be fired from their jobs without cause in a majority of the states, and the federal Employment Non-Discrimination Act has still, shockingly, not become law after two decades of trying. Suicide rates among gays are far higher than the average; homelessness is, too. Men who have sex with men also face the harrowing resurgence of HIV/Aids over the last decade; yet, gays of my generation, who came of age long after the first phase of the epidemic, mostly see Aids as either a historical phenomenon or a sub-Saharan one.

Today, however, one and only one gay rights issue has crowded out all others. To such a degree that when we finally, inevitably, win the right to wed, gay advocacy may lose much of its momentum or peter into nothingness.

I don't mean to minimize marriage. It matters tremendously. But it matters because of its public, national character, not because of the small-scale, individualist assertions of the largely upper-class mainstream marriage movement. Nancy Cott, author of the book Public Vows: A History of Marriage and the Nation and a lead witness in Perry, has shown that the marriage in America, far from the eternal institution that homophobes imagine, has mutated considerably – but it has done so along with our understanding of citizenship and public life.

Even before the American Revolution, the colonists rejected Anglican tradition, in which the church oversaw questions of matrimony, and established marriage as a civil and not religious institution. In the 19th century, states used marriage laws as punitive instruments to discriminate against disfavored minorities, especially Asian immigrants on the west coast.

But as conceptions of citizenship matured in the 20th century, marriage laws did, too. Married women, who once had no legal independence separate from their husbands, won the right to own property or enter contracts under their own names. Interracial couples, of whom Barack Obama's parents were one, won the right to marry in any state in the 1960s. Other disadvantaged populations, from prisoners to the mentally disabled, have latterly been guaranteed the right to marry.

So, marriage is not just a nice private affair to which gay people want admission. It is a bedrock constitutional right, and perhaps the most basic sign of equality in this country.

The history of marriage in America helps explain why Obama erred in believing (or so he says) that civil unions, which supposedly guarantee the protections of a marriage contract without the name, were good enough for gays. Other countries have implemented parallel institutions, such as Britain's civil partnership or France's PACS; but in the United States, only marriage is marriage, and nothing else will do. And more radical gays, who reject the entire institution of marriage or see homosexuality and monogamy as incompatible (a position to which I'm hardly unsympathetic), should remember that the bridal chorus from Lohengrin and the Crate & Barrel registry are not really what's at issue here.

The question is not, as Biden said last week, "Who do you love?" The question is: "Who is American?"

Yet, if little of the rhetoric around marriage equality concerns the stuff of marriage rights – taxes, inheritance, social security, immigration – even less of it expresses this civic character of the institution. Instead, arguments for same-sex marriage in America have taken on a perturbing libertarian strain: the government has no business in my private affairs; my marriage has no effect on yours. You can hear this in the outrageous refrain "If you don't like gay marriage, don't get gay married" – a blinkered and fantastically antisocial understanding of the value of marriage, as if the intolerance of homophobes was hopelessly permanent, rather than something we should all be working to change.

And when the movement for gay equality reduces itself to an isolationist platform, you can be sure of what comes after. Sooner or later, gays will win the right to marry in the United States. It is a certainty. But discrimination, intolerance, disease: these will be with us for a while, and if we make marriage into mere private affirmation rather than public endeavor, it's hard to see how we can combat these other scourges together, once the weddings are over; in fact, it's hard to see that the word "together" will signify anything at all.

The president is getting a lot of credit this week for striking a blow for equality. But if marriage is the only battle we win, that will not be any equality worth the name.

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